Problem: Quantities $r$ and $s$ vary inversely. When $r$ is $1200,$ $s$ is $0.35.$ What is the value of $s$ when $r$ is $2400$? Express your answer as a decimal to the nearest thousandths.
Answer: Since $r$ and $s$ vary inversely, $r\cdot s$ must be a constant.  Thus $1200\cdot .35 = s \cdot 2400 \Rightarrow s = \frac{.35}2 = \boxed{.175}$.